Resistance of Microorganisms to Decontamination

Microorganisms differ greatly in their ability to tolerate destruction by physical or chemical means. As demonstrated in Fig. 1 vegetative bacteria, viruses, fungi, and mycobacteria are often considered the least resistant to decontamination and can usually be reduced to a sanitary level by sanitizers or destroyed by disinfection methods. Bacterial endospores and other protective shell structures such as oocysts and eggs are the most resistant type of pathogen and are only killed by sterilization processes. The potent power of physical or chemical sterilization processes destroys the robust protective layers of these endospores and shell structures, destroying their genomes (Lai et al., 2003; Riesenman & Nicholson, 2000; Setlow, 2006; Swenson, 2012).
Endospores are the most resistant type of pathogen and extreme sterilization methods are required to destroy them. Endospores are a pathogen’s method of surviving in extreme conditions. Endospores ofBacillus species have demonstrated the ability to resist and survive extreme conditions, such as highly acidic environments, prolonged exposure to high temperatures, non-ionizing and ionizing radiation, as well as strong antibiotics including ampicillin, cephalothin and oxacillin (Berg & Grecz, 1970; Byrne, Dunne, & Bolton, 2006; Clavel, Carlin, Lairon, Nguyen-The, & Schmitt, 2004; Schlegelova, Babak, Brychta, Klimova, & Napravnikova, 2003; Setlow, 1995).
Spores have multiple protective layers, which act as barriers, and accounts for their extreme resistance to decontamination. The first barrier is the external layer, which consists of either an exosporium or a spore coat. The exosporium is composed of a number of different proteins, while a spore coat consists of both proteins and glycoproteins. The external layer has the ability to filter and detoxify many environmental contaminants (Lai et al., 2003; Setlow, 2006). The barrier beneath the external layer is the cortex, which is formed of a thick layer of peptidoglycans. The cortex protects the core from destruction by organic solvents. The third barrier, situated beneath the cortex, is the cell wall, which is again composed of peptidoglycans. Beneath the cell wall is a cell membrane, which safeguards the central core. The final barrier is the central core, which consists of small acid-soluble binding proteins (SASP) that protect the DNA. Spores are able to survive for many years until favorable conditions arise, at which point they can then develop into vegetative cells (Driks, 2002; Riesenman & Nicholson, 2000; Setlow, 2006).
Protozoa are microscopic unicellular organisms, which are widespread in almost every habitat. Some species of protozoa are commensal and are not pathogenic to their hosts, whereas others are pathogenic and may cause a range of diseases from mild in severity to life-threatening, such as malaria. Infection from protozoa can be caused by contaminated water, food, and soil via sporulated oocysts passed in the feces of the host. Protozoal oocysts, which are an essential stage of the life cycle of protozoa ((CDC), 2004; Yaeger., 1996) have a high level of resistance to chemical and physical decontamination treatments, due to their protective membrane or hardy cell wall that is composed of two layers of over 90% protien. The outer layer of the oocyst wall contains mainly lipids-free quinone-tanned proteins, whilst the inner layer consists of a lipid-protein matrix (Mai et al., 2009).
Helminth cause parasitic infections that lead to the tropical disease, Helminthiasis. The female helminth worm deposits the eggs into the host in a process known as oviposition. Adult helminth can deposit up to 700,000 eggs, six times a day. The helminth eggs are highly resistant to chemical and physical decontamination methods, because of their layered structure which provides resistance under several conditions. There are three basic layers which consist of an outer proteinic layer, followed by a chitinous layer, and then an inner lipoidal layer (Jimenez, 2007). The persistence of helminth ova is the main constraint for the reuse of water and wastewater (WHO, 2006).
Fungi and fungal spores exhibit high resistance to decontamination treatments (Ma & Bibby, 2017). Waterborne fungi are considered responsible for environmental problems such as turbidity, odor and mycotoxin emissions, in addition to waterborne diseases caused byAspergillus spp . and Penicillium spp . (Curtis, Lieberman, Stark, Rea, & Vetter, 2009; Hageskal, Knutsen, Gaustad, de Hoog, & Skaar, 2006; Oliveira, Barreto Crespo, & Pereira, 2020; Pereira et al., 2009).
Bacteria can be classified as Gram positive (GP) or Gram negative (GN), based on the structure of the cell wall. The cell wall of GP bacteria is characterized by a thick peptidoglycan layer with no outer lipid membrane, while the peptidoglycan layer is thin in GN bacteria, and supported with an outer lipid membrane (Gram, 1884). 90-95% of GN bacteria are pathogenic and are often implicated in severe disease, such as Cholera caused by Vibrio cholerae , whilst most GP bacteria are non-pathogenic (Abe et al., 2010; Alexandraki & Palacio, 2010). Although these pathogenic GN bacteria show more resistance to antibiotics than GP strains, they are more susceptible to decontamination methods and can be easily decontaminated. Comparatively, GP bacteria have more resistance to decontamination methods, but tend to be less harmful to humans (Howie, Alfa, & Coombs, 2008; Traverse & Aceto, 2015).
Mycobacteria has its name derived from the latin word myco , which refers to fungus, because mycobacteria have been observed to grow in a mold-like manner when cultivated in laboratories. Mycobacteria are responsible for serious diseases in humans, such as tuberculosis and leprosy (Ryan & Ray, 2004). Mycobacteria show a high level of resistance to chemical and physical decontamination methods due to their cell wall, which is composed of hydrophobic mycolic acid and peptidoglycan layers that are interconnected by a highly branched polysaccharide (arabinogalactan), which represents about 80% of the cell wall (Alderwick, Harrison, Lloyd, & Birch, 2015; Jackson, 2014).
The extracellular form of a virus that spreads from one organism to another is called a virion. In contrast to other microorganisms, viruses can not be considered as living organisms as they lack their own metabolism. A virion consists of a viral genome (containing both DNA and RNA), which is enclosed in a protein capsid that provides protection to the genome. Viruses can be classified into two types; enveloped and non-enveloped, according to their cell membrane. Viruses are referred to as enveloped when the protein capsid is surrounded by a membrane (“envelope”), which is composed of a lipid bilayer studded with virus-coded proteins in the shape of spikes or knobs, called peplomers. The role of the biological membrane is to protect the virus against attack from the host immune system. Viruses without a membrane are known as non-enveloped or “naked” viruses. Contrary to what one would expect, non-enveloped viruses are the most resistant to decontamination methods, and smaller non-enveloped viruses are more resistant than larger ones. This is because outer lipid bilayer “envelopes” can be easily neutralized by various chemical and physical agents, and a virion is only infectious if it is fully intact. Hence, if the envelope is destroyed, a virion is no longer infectious (Gelderblom, 1996; Howie et al., 2008; Traverse & Aceto, 2015).